Poly (2-hydroxyethylmethacrylate) membranes for electrochemical use and the manufacture thereof

ABSTRACT

The invention relates to a semi-permeable membrane for lead-acid batteries. The membrane has a microporous structure comprising a first polymer and a second polymer closely mixed together, the first polymer being poly (2-hydroxyethylmethacrylate), known as reticulated poly HEMA, characterized by the fact that the second polymer comprises one polymer or a mixture of several different polymers chosen from among polyoxyethylene glycol, polyvinylpyrrolidone, polyacrylic acid, polymethacrylic acid, copolymers of acrylic and methacrylic acid, polyacrylates and polymethacrylates of alkyl, and the copolymers of the acrylates and methacrylates of alkyls.

This is a division of application Ser. No. 860,410, filed Dec. 13, 1977.

The present invention relates to the preparation ofpoly(2-hydroxyethylmethacrylate) membranes which can be used as ionseparators in mediums in which there are electrochemical reactionstaking place, in particular in lead-acid storage cells. It also relatesto membranes made by method of preparation.

The diffusion of antimonyl SbO⁺ ions coming from the corrosion of theelectrode grids, from the positive PbO₂ to the negative Pb, causes thedeposit of antimony on this latter electrode, this having the effect ofreducing the charge efficiency of the storage cell by lowering thepolarisation of the hydrogen. The result of this is a reduction of theperformance of the cell, mainly of its storage life.

The presence of antimony, in a concentration of 4% to 6% in thecomposition of the electrode grids is partially motivated by the need toobtain good fluidity in casting as well as a sufficient hardness of thePb--Sb alloy after solidification. However, the replacement of antimonyby calcium, at ratios lying between 0.065 and 0.090% has not made itpossible to solve all these problems, since besides its mechanicalaction mentioned above, antimony plays a complex electrochemical part inthe operation of the active material (PbO₂), in particular it enablesthe α variety of PbO₂ to pass to the β variety of PbO₂ which is moredivided and active. The problem of the confinement of the antimony inthe positive compartment therefore still remains, hence the advantage ofbeing able to have membranes available which are selective with respectto SbO⁺, the stable form of antimony in a highly acid oxidising medium(H₂ SO₄ at 9N).

The majority of work on the electrolyte of lead-acid cells has generallybeen on other aspects of the function of the separators (chemicalresistance, immobilization of the electrolyte and electricconductivity), the selective screen function appearing only secondarily.

The work on porous separators (charged or uncharged PVC) which is thesubject of the article "Role et utilisation des membranes anelectrochimie et en electrotechnique" by G. Feuillade and M. Jacquier inEntropie, 49 (January, 1973), p 21, and of the article "Dic Wirkung vonSeparatoren auf die Antimonwanderung in Bleibatterien" Electrochim. Acta(1965), 9, p 55 by Zehende, Hermann, and Leibssle or on gelledelectrolytes which are the subject of the article "BatteryMaterials"--Noyes Data Corp. (1970), p 7, 24 104 by P. Conrad hasgenerally developed on these lines and a systematic search for selectiveseparators with very fine micro porosity has been practicallynon-exisent. Even research workers concerned with transfer of antimonyhave considered only porous separators which only have tortuosityeffects that are strictly geometrical.

It is therefore important to known how to prepare non-porous membranesor membranes with very fine microporosity whose separation effect is dueto highly selective chemical or ionic interactions and through which H⁺ions pass preferably by dissolution and by diffusion rather than byviscous flow. Since these membranes must have a relatively long servicelife in the medium considered, it is necessary that the polymer used fortheir preparation should resist well acid hydrolysis and oxidation.

Further, these membranes must have sufficient mechanical properties,both in the dry state and in the wet state, for them to be easilyhandled without danger of tearing during their transfer in the acidmedium. This is why it is particularly interesting to known how toprepare membranes supported by a macroporous unit made of polypropylene,polyvinyl chloride, fibre glass or another resistant material.

The invention relates mainly to poly (2-hydroxyethylmethacrylate) (inshort poly-HEMA) membranes, and to a method of producing them. They areintended for the use described hereinabove and preferred embodimentshave the following properties:

A very finely microporous structure in which the average diameter of thepores is less than twenty of so A). This structure is characterized inthat it allows the H⁺ ions and consequently electric current to passthrough the membrane, while stopping the passage of the SbO⁺ ions whichare responsible for the reduction of the charging efficiency of thecell. More precisely, this structure is such that the electricalresistance of the membrane, immersed in a 9N aqueous solution of H₂ SO₄is less than 150 mω/cm² and that the permeability of the membrane to Sb₂O₃ remains less than 0.1 mg/cm² - hour.

High chemical stability allowing the membranes to resist acid hydrolysisand oxidation and to maintain their electrical conductivity and theirselectivity with respect to the SbO⁺ ions for a long time.

A relative insensitivity to temperature allowing the membranes to keeptheir mechanical properties in a temperature range lying between -20° C.and 70° C.

strong adhesion to a macroporous support whose function is to supportthe membranes and to make them easy to handle. The membrane and thesupport form, in these conditions, a composite membrane whose mechanicalproperties (flexibility, breaking strength, etc . . .) are generallygreater than those of the membrane and of the support consideredseparately.

The invention provides a semi-permeable membrane with a microporousstructure comprising a first polymer and a second polymer intimatelymixed together, the first polymer beingpoly(2-hydroxy-ethylmethacrylate) called reticulated poly HEMA, whereinthe second polymer comprises one polymer or a mixture of severaldifferent polymers chosen from among the following substances:polyoxyethylene glycol, polyvinylpyrrolidone, polyacrylic acid,polymethacrylic acid, copolymers of acrylic or methacrylic acid,polyacrylates and polymethacrylates of alkyls, and copolymers ofacrylates and methacrylates of alkyls.

The invention also provides a method of production of such membranes.

Other advantages of the method in accordance with the invention and ofthe membranes which are thereby formed will become apparent from thefollowing description, given by way of purely illustrative examples.

Membranes in accordance with the invention are prepared by polymerising,at ambient temperature or at a temperature higher than ambienttemperature, by a photochemical method, the 2-hydroxyethylmethacrylate(in short HEMA) in the presence of a plasticizer and of a solublepolymer, or better of a mixture of a soluble polymer and of an insolublepolymer in a medium formed by an aqueous solution of 9N H₂ SO₄. Thepresence of the plasticizer makes the structure of the membranes moreslack, due to the increase in the intermolecular distances of thepoly-HEMA chains and of the reduction of the reticulation number. Theresult of this is a better electrical conductivity of the membrane. Thepresence of the plasticizer is also necessary to impart elasticity andresilience to the membrane which would otherwise be brittle in the drycondition. The addition of a polymer to the HEMA solution increases itsviscosity and thus makes it possible to use the usual technique for theproduction of membranes, which consists of applying the solution on aplane support by means of a knife. The presence of a polymer furtherfacilitates the polymerisation of the HEMA in a thin layer of 50μ to200μ thick.

In the case where the polymer added to the HEMA solution is soluble in9N aqueous H₂ SO₄, it can diffuse partially on the outside of themembrane and increase its porosity and consequently its electricalconductivity.

However, this membrane cannot diffuse totally on the outside of themembrane, since the HEMA has a tendency to interpolymerise with it andto graft onto it during the irradiation by ultra-violet rays. Thepresence of this polymer in the membrane nevertheless allows theelectrical conductivity of this membrane to be increased because itincreases the expansion rate of the membrane.

In the case where the polymer added to the HEMA solution is insoluble inthe 9N H₂ SO₄ aqueous solution, the polymer remains completely in themembrane. Therefore, this polymer should be chosen among those whichmake it possible to increase the mechanical strength and the chemicalstability of the membrane. This polymer should be chosen preferably fromamong those which increase the adhesion of the membranes to thereinforcing supports used (polypropylene felts or fibre glass felts).

The incorporation in the HEMA solution of a mixture of these two typesof polymers makes it possible to impart to the membrane simultaneouslyacceptable electrical conductivity and good chemical stability.According to the composition of the mixture of these two types ofpolymers, one or the other of the properties mentioned is preponderant.

The following plasticizers, used alone or mixed, are suitable for thepreparation of membranes in accordance with the invention:

dihydric alcohols, such as ethylene glycol, propylene glycol,trimethylene glycol, diethylene glycol, triethylene glycol orpolyoxyethylene glycols;

Trihydric alcohols such as glycerol of triethanolamine; and

2-pyrrolidone or methylpyrrolidone.

The proportion of plasticizer added to the HEMA lies between 5% and 40%by weight with respect to the HEMA.

Among the polymers which it is interesting to add to the plasticizingHEMA mixture for the reasons set forth hereinabove, the distinctionshould be made between those which are soluble in the 9N aqueous H₂ SO₄medium and those which are insoluble in this medium. The followingpolymers come within the first category:

polyoxyethylene glycol and polyvinylpyrrolidone.

The following polymers come with the second category:

polyacrylic and polymethacrylic acids, copolymers of acrylic acid and ofmethacrylic acid, and alkyl polyacrylates and copolymers of alkylacrylates and methacrylates.

The proportion of polymer which it is recommended should be added to theHEMA lies between 1% and 30% by weight.

As photosensitive promoters, use can be made of uranyl salts such asuranyl nitrate and uranyl acetate, organic compounds such as benzoin,sodium p-toluene sulfinate, in the presence of methylene blue, sodiumanthraquinone-2-sulfonate and others.

To reinforce the membrane, a felt or a very permeable fabric is usedwhich is made of a material resistant to the action of H₂ SO₄ and ofoxygen. In some cases, it can be an advantage to be able to preparethese membranes so that the greater part of the poly HEMA will besituated on one side of the felt or of the fabric. More precisely, thereinforced membrane must have a thin microporous film of poly HEMA onone surface and a macroporous layer mostly constituted by the felt orthe fabric on the other surface. Such a disposition can be obtained asfollows:

The felt or the fabric is deposited on a sheet of TEFLON or of TERPHANEwhich itself covers a suction support. The felt or the fabric is thenimpregnated with the HEMA solution by means of a brush or of a rollerpreferably made of silicone rubber). The solution is allowed to diffusefor a few minutes towards the surface of the felt or of the fabric whichis in contact with the TEFLON sheet, before exposing the whole toultra-violet radiations. To facilitate the impregnation of the felt, theHEMA solution can be diluted in a volatile solvent such as methanol,ethanol, acetone and others. If on the contrary the membrane is requiredto be distributed evenly in the felt or the fabric, a solution havinghigher viscosity is used and the impregnated felt or fabric is exposedto ultraviolet radiation immediately after its impregnation so that thesolution cannot gather together at the felt-TEFLON or fabric-TEFLONinterface.

Another method making it possible to obtain the above-describedstructure, which comprises at least two layers, consists in spreading bymeans of an applicator a fairly thick HEMA solution on a TERPHANE sheetcovering a suction support and in laying the felt on the layer thusformed. Due to the high viscosity of the solution, the felt penetratesonly partially into the solution. The whole is then exposed toultra-violet radiation. If the viscosity of the solution is reduced, thefelt or the fabric enters more deeply into the solution and thus, afterpolymerisation of the HEMA, a more uniform distribution of the membraneis obtained inside the felt or fabric.

It is important for the membrane to adhere to the filter or to thefabric. It is observed that the adhesion decreases with an increase inthe concentration of soluble polymer in the 9N H₂ SO₄ added to the HEMA.The limiting concentration which is not to be exceeded depends on themolecular mass of the polymer.

The electrical resistance of the membranes which are mentioned in theexamples hereinbelow was measured as follows:

A sample of membrane was placed between two compartments of ameasurement cell filled with a 9N aqueous H₂ SO₄ solution and thedifference in potential between the two surfaces of the membrane wasmeasured as a function of the current which was passing through themembrane. The resistance determined in the same way when there is nomembrane was subtracted from the resistance thus obtained. The DCinjection electrodes were made of lead and the electrodes used formeasuring the difference in potential between the two surfaces of themembrane were made of mercury and mercurous sulfate.

The permeability of the membranes to the Sb₂ O₃ was measured as follows:

A sample of membrane was placed between the two compartments of a cellone of whose compartments contained a 9N aqueous solution of H₂ SO₄saturated with Sb₂ O₃ and the other of whose compartments contained a 9Naqueous H₂ SO₄ solution. The quantity of Sb₂ O₃ which penetrated intothis compartment was determined by periodic drawing off of a smallquantity of the solution and by dosing of the Sb₂ O₃ by means ofpotassium iodide-ascorbic acid reagent. This reagent turns yellow in thepresence of trivalent antimony. The intensity of this colouring wasdetermined with an ultra-violet spectrophotometer, in accordance withthe method described by A. Elklind, K. H. Gayer and D. F. Boltz, inAnalytical Chem. Vol. 25 n° 11 (1953 1744).

EXAMPLE 1

A solution having the following composition was used:

84% HEMA;

10% glycerine;

4% PRIMAL AC 34 (ROHM and HAAS acrylate-methacrylate copolymer; and

2% Uranyl nitrate.

This solution was applied by means of an applicator on a plane surface(for example a suction support) covered with a TERPHANE sheet (ethylenepolyterephthalate) and the layer of solution thus obtained was exposedfor 2 to 5 minutes to ultra-violet radiation from a mercury vapour andmetallic iodide lamp (PHILIPS HPM 12, 400 watts) placed at a distance ofabout 30 cm from the surface. Five to ten minutes after this exposure,the membrane was unstuck from the TERPHANE sheet. Its thickness was 160microns.

After immersion for 48 hours in a 9N aqueous H₂ SO₄ solution, themembrane had an electrical resistance of 140 mΩ/cm² and a permeabilityto Sb₂ O₃ of 0.026 mg/h-cm².

After immersion for 97 days in a 9N aqueous solution of H₂ SO₄maintained at 65° C. with oxygen bubbling through it, the electricalresistance of the membrane was 95 mΩ/cm² and its permeability to Sb₂ O₃was 0.028 mg/h-cm².

EXAMPLE 2

A membrane was prepared whose thickness was 200 microns in accordancewith the method set forth in example 1 using a solution whosecomposition was as follows:

78% HEMA;

18% ethylene glycol;

3% PRIMAL AC 34; and

1% Uranyl nitrate.

After two days of immersion in a 9N aqueous solution of H₂ SO₄, themembrane had an electrical resistance of 130 mΩ/cm² and a permeabilityto Sb₂ O₃ of 0.013 mg/h-cm².

After immersion for 47 days in a 9N aqueous solution of H₂ SO₄maintained at 65° C., with oxygen bubbling through it, the electricalresistance of the membrane was 95 MΩ/cm² and its permeability to Sb₂ O₃was 0.015 mg/h-cm².

EXAMPLE 3

A membrane with a thickness of 200 microns was prepared in accordancewith the method set forth in example 1, using a solution whosecomposition was as follows:

75% HEMA;

21% ethylene glycol;

3% PRIMAL AC 34; and

1% Uranyl nitrate.

After two days immersion in a 9N aqueous H₂ SO₄ solution, the electricresistance of the membrane was 112 mΩ/cm² and its permeability to Sb₂ O₃was 0.014 mg/h-cm².

After immersion of the membrane for 47 days in a 9N aqueous solution ofH₂ SO₄ maintained at 65° C. with oxygen bubbling through it, the saidmagnitudes are respectively μmΩ/cm² and 0.014 mg/h-cm².

EXAMPLE 4

In accordance with the method set forth in the Example 1, a membrane wasprepared which had a thickness of 150μ, using a solution whosecomposition was:

68% HEMA;

10% polyoxyethylene glycol of molecular weight 15,000;

1% PRIMAL AC 34;

20.3% ethylene glycol; and

0.7% Uranyl nitrate.

The electrical resistance of the membrane was 80mΩ/cm² and itspermeability to Sb₂ O₃ was 0.040 mg/h-cm².

EXAMPLE 5

A solution having the following composition was prepared:

69% HEMA;

29% ethylene glycol;

1% Polyoxethylene glycol having a molecular weight of 2000; and

1% Uranyl nitrate.

A polypropylene felt (VILEDON L 32237 F thickness 200μ) was impregnatedwith this solution so that a mass of 24 grams of this solution is spreadout over an area of 120 cm² of felt. For the solution to be distributedhomogenously, it was diluted with ethanol. Impregnation was effected inaccordance with the method described hereinabove.

The thickness of the reinforced membrane obtained after the impregnatedfelt had been exposed for a few minutes to ultraviolet radiation was 290microns. Its electric resistance was 155mΩ/cm² and its permeability toSb₂ O₃ was 0.048 mg/h-cm².

EXAMPLE 6

A solution having the following composition by weight was prepared:

68.2% HEMA.

25.7% ethylene glycol;

5.5% PRIMAL AC 34; and 0.6% Uranyl nitrate.

This solution was spread by means of an applicator on a plane suctionsupport covered with a TERPHANE sheet. Thus, a layer having an equalthickness of about 200 microns was obtained on which a polypropylenefelt (VILEDON L23237 F) was laid which penetrated partially into thesolution. The whole was then exposed for a few minutes to ultra-violetradiation as described in example 1.

The total thickness of the reinforced membrane after polymerisation ofthe HEMA was 280 microns. The electrical resistance and the permeabilityto Sb₂ O₃ of the membrane were respectively 95mΩ/cm² and 0.087 mg/h-cm².

After an immersion of the membrane for 41 days in a 9N aqueous solutionof H₂ SO₄ maintained at 65° C. with oxygen bubbling through it, thepreceding characteristics become respectively 70mΩ/cm² and 0.075mg/h-cm².

It must be understood that the invention is is no way limited to theexamples described, but that it covers on the contrary all variantsthereof.

What is claimed is:
 1. A method of producing a membrane consisting inimmersing, in an electrolyte of 9N aqueous H₂ SO₄, a film prepared froma casting solution which comprises:a monomer comprising2-hydroxyethylmethacrylate in a proportion by weight of 50 to 90%; aplasticizer of poly(2-hydroxyethylmethacrylate) in a proportion byweight lying between 5 and 40% chosen preferably from among thefollowing substances: ethylene glycol, propylene glycol, trimethyleneglycol, diethylene glycol, triethylene glycol, polyoxyethylene glycol,glycerol, 2-pyrrolidone, and methylpyrrolidone; a polymer or a mixtureof polymers in a proportion by weight lying between 1 and 30% at leastone of which is soluble in the 9N aqueous H₂ SO₄ electrolyte, thepolymer or polymers of the mixture being chosen from among the followingpolymers: polyoxyethylene glycol, polyvinyl pyrrolidone, polyacrylicacid, polymethacrylic acid, copolymers of acrylic and methacrylic acid,alkyl polyacrylates and alkyl polymethacrylates, and copolymers of alkylacrylates and alkyl methacrylates; and a photosensitive promoter in aproportion by weight lying between 0.2 and 2% chosen from among thefollowing substances: uranyl nitrate, uranyl acetate, benzoin, andsodium anthraquinone-2-sulfonate, said 2-hydroxyethylmethacrylateentering in the composition of the casting solution being polymerizedphotochemically in the presence of the other constituents of thesolution.
 2. A method according to claim 1 comprising the steps ofcasting the casting solution in the form of a liquid layer on a sheet ofpolytetrafluoroethylene or of polyethylene terephthalate covering asuction support; exposing the solution to ultraviolet radiation, atambient temperature or at a temperature higher than ambient temperatureto polymerize the 2-hydroxymethylmethacrylate in the presence of theother constituents of the solution, and then immersing this film in the9N aqueous H₂ SO₄ electrolyte.
 3. A method according to claim 2,including a further step of depositing a felt or a permeable fabric onthe layer of solution before exposing it to ultra-violet radiation, soas to obtain a film reinforced by the felt or the fabric, prior toimmersing the whole in an 9N aqueous H₂ SO₄ electrolyte.
 4. A methodaccording to claim 1 comprising the steps of depositing a felt or apermeable fabric on a suction support which is itself covered with asheet of polytetrafluorethylene or of polyethylene terephthalate,impregnating the felt or the fabric with the casting solution, allowingthe solution time to diffuse towards that surface of the felt or of thefabric which is in contact with the sheet, exposing the whole toultra-violet radiation and then immersing the film thus obtained andadhering to the felt or to the fabric in an the 9N aqueous H₂ SO₄electrolyte.
 5. A method according to claim 4 wherein the exposure toultra-violet radiation is effected immediately after the impregnation ofthe felt or of the fabric by the solution.
 6. A lead-acid storage cellcomprising a positive electrode, a negative electrode, a separator and a9N aqueous sulphuric acid electrolyte, wherein said separator at leastpartially comprises a semi-permeable membrane with a microporousstructure comprising a first polymer and a second polymer intimatelymixed together, the first polymer being poly(2-hydroxyethylmethyacrylate) and the second polymer comprising onepolymer or a mixture of several different polymers selected from thegroup consisting of polyoxyethylene glycol, polyvinylpyrrolidone,polyacrylic acid, polymethacrylic acid, copolymers of acrylic ormethacrylic acid, alkyl polyacrylates, alkyl polymethacrylates, andcopolymers of alkyl acrylates and alkyl methacrylates, saidsemi-permeable membrane being produced by immersing, in an electrolyte,a film prepared from a casting solution which comprises:a monomercomprising 2-hydroxyethylmethacrylate in a proportion by weight of 50 to90%; a plasticizer for said poly(2-hydroxyethylmethacrylate) in aproportion by weight of between 5 and 40%; said one polymer or saidmixture of several different polymers in a proportion by weight between1 and 30%, at least one of which is solbule in said electrolyte; and aphotosensitive promoter in a proportion by weight of between 0.2 and 2%selected from the group consisting of uranyl nitrate, uranyl acetate,benzoin and sodium anthraquinone-2-sulfonate, said2-hydroxyethylmethacrylate entering into the composition of the castingsolution being polymerized photochemically in the presence of the otherconsituents of said casting solution.